The present invention generally relates to machines and methods capable of evaluating threaded fasteners. The invention particularly relates to machines and methods of performing torque-tension evaluations of prevailing-torque threaded fastener assemblies that comprise a locking feature, resulting in the fastener assembly exhibiting a prevailing torque during assembly.
The fastener industry has utilized various inspection procedures for qualifying prevailing-torque threaded fasteners. These procedures typically specify certain parameters of particular interest, including but not limited to drive torque, thread torque, prevailing torque, tension, etc. As used herein: “tension” will refer to the tensile loading of a threaded bolt caused by driving a threaded nut onto the bolt such that the bearing surfaces of the nut and bolt are drawn toward each other and the nut-bolt assembly generates a clamping load therebetween; “drive torque” will refer to the torsional resistance to assembling a threaded nut onto a threaded bolt, measured as the torque required to drive the nut onto the bolt and including the torque required to develop tension in the bolt as the bearing surfaces of the nut and bolt are drawn toward each other during assembly; “thread torque” will refer to the torsional resistance to assembling a threaded nut onto a threaded bolt, measured as the torque required to prevent the bolt head from rotating and including the torque required to develop tension in the bolt as the bearing surfaces of the nut and bolt are drawn toward each other during assembly; “final assembly torque” will refer to drive and/or thread torque; “prevailing torque” will refer to the torsional resistance to assembling a threaded nut onto a threaded bolt, measured as the torque required to prevent the bolt head from rotating while there is no tension in the bolt as the bearing surfaces of the nut and bolt are drawn toward each other during assembly or away from each other during disassembly; and “bolt-through” will refer to the condition during assembly of a threaded nut onto a threaded bolt at which the end of the bolt opposite its head and bearing surface is flush with the exit end of the nut (i.e., the end opposite the bearing surface of the nut).
Standard inspection procedures for qualification of some prevailing-torque threaded fasteners often impose various requirements on the above and other parameters. Some of these requirements and problems that may be encountered are briefly summarized below.
Standard inspection procedures sometime require the measurement of prevailing torque (measured before the nut-bolt assembly develops any clamping tension) and final assembly torque (measured when the nut-bolt assembly develops the final clamping tension). These procedures generally require stringent percent-of-point accuracy for both the prevailing torque measurements and the final assembly torque measurements. The ratio of the magnitude of prevailing and final assembly torque measurements can be 1:10 for a given nut-bolt assembly. It is also usually desirable that equipment used to perform torque measurements are capable of testing a range of fastener sizes and material strength grades, which may further necessitate a 1:10 ratio of torque measurements between the torque levels required for nut-bolt assemblies of the smallest size and/or lowest strengths relative to the torque levels required for nut-bolt assemblies of the largest size and/or highest strengths. It is difficult with a single sensor to economically and accurately measure torque over a range that, based on the foregoing, may encompass a torque ratio of 1:100.
Standard inspection procedures usually require that one member of a nut-bolt assembly is constrained from rotating with a constraining tool, while the other member is rotationally driven by a socket or other suitable drive tool. Due to the helical nature and geometry of screw threads, the relative rotation between the driven and constrained members of a nut-bolt assembly will cause relative linear motion between the members of the assembly as the members axially translate relative to each other. Unless the relative linear position of the drive tool and constrained member is accurately coordinated with their relative rotational position, there will be dragging (friction) forces and/or torques introduced into the nut-bolt assembly that can introduce errors into the test measurements. These forces can occur if the driven member is able to slide within or otherwise shift relative to the drive tool, the constrained member is able to slide within or otherwise shift relative to the constraining tool, or the drive tool rubs the bearing surface of the constrained member. In addition, if the relative linear position of the drive tool and constrained member is not coordinated with their relative rotational position, it may be necessary for the constraining tool or drive tool to be deep enough to accommodate the change in the relative linear position between the members of the nut-bolt assembly.
Standard inspection procedures may also require the measurement of temperature at the thread interface of a nut-bolt assembly to ensure that the temperature remains within a prescribed allowance from room temperature. One way to monitor this temperature is to continuously observe the thread temperature where the bolt thread exits the nut with the use of an optical measurement instrument. However, because the linear distance between the bearing surfaces of the bolt and nut changes as the nut-bolt assembly is driven rotationally to assemble or disassemble, a “moving target” is presented to the optical measurement instrument unless steps are taken to continuously adjust the aim of the instrument or to fix the targeted bolt thread where it exits the nut. Another obstacle is that the nut is usually constrained or driven by a socket, which often completely envelopes the nut including the targeted bolt thread where it exits the nut, obscuring the bolt thread from optical temperature measurement.
Standard inspection procedures also sometimes require that torque measurements are summarized and recorded at defined intervals during a test. Some of those intervals are defined relative to the bolt-through position where the end of the bolt opposite its head is flush with the exit end of the nut (the surface opposite its bearing surface), often taken as when the bolt starts to protrude from the exit end the nut. Observing a bolt-through event in real-time is problematic because the tool or adapter that constrains or drives the nut often completely obscures the event if simple measurement or detection methods are used.